Composite parts have become commonly used throughout industry because of their engineering qualities, design flexibility and low weight. In particular, honeycomb composite parts having a honeycomb core bonded between two composite face sheets provide excellent strength and stiffness to weight ratios that make them particularly valued and widely used in the aerospace industry. However, despite the recognized benefits of this type of parts and their wide use, they have been relatively expensive, in part because the manufacturing processes for producing these parts were clumsy and difficult to use, resulting in high reject and rework rates for composite parts.
The standard process for making composite parts included laying up a tool-side skin, usually several plies of resin-impregnated fiberglass or graphite cloth, on the surface of a tool known as a "bond assembly jig" or BAJ. If the part was to have a honeycomb core, the honeycomb material was cut and fitted onto the tool-side skin and the assembly is covered with a vacuum bag from which the air was withdrawn with a vacuum source. The bagged assembly was inserted into an autoclave and reconnected to the vacuum source while it was heated to cure the resin in the tool-side skin plies and bond the honeycomb to the skin. The bagged assembly was then removed from the autoclave and unbagged.
The part was then machined to shape the honeycomb core to the desired configuration. The machining was performed by an CNC machine tool such as a gantry mounted robot, but it was not possible to perform this machining operation with the part on the BAJ because it had no means for indexing to a machine bed, and there are no provisions for holding the lay-up assembly on the tool face of the BAJ. More importantly, there was no relief in the tool face into which the cutters could project when edge routing, drilling, or other cutting operations. Instead, the part was broken out of the BAJ, and transferred to another tool known as a "bond mill fixture" or BMF. The BMF had a part support surface that was designed to have the same profile as the BAJ and also was provided with vacuum ports and hold down mechanisms intended to hold the part in place on the BMF while the honeycomb core material was machined to sculpt it to the desired shape. The BMF also had index features for accurately locating it on the machine tool bed, and it had an accurately machined "A" datum plane for orienting the part support surface of the BMF relative to the machine tool bed to facilitate accurate machining by CNC machine tools.
The partially made part was removed from the BMF and the honeycomb was cleaned to remove dust from the cells. The cleaned part was repositioned onto the BAJ where it was reattached with clamps and hold-down devices. The plies for the bag-side skin were laid over the honeycomb core and were recovered with another vacuum bag. The BAJ was reinserted back into the autoclave where the bag-side skin was bonded to the honeycomb core. After curing, the cured part was again removed from the BAJ and repositioned back onto the BMF for final trim.
That time consuming, costly and error-prone process has been replaced by a far more efficient and accurate process, described in U.S. patent application Ser. No. 08/629,120 filed on Apr. 8, 1996 by Dwight Engwall and entitled "Dual Purpose Lay-Up Tool". It provides a single tool having a face sheet with a facing surface configured to a desired shape of one surface of a part to be made on the tool. A groove in the face sheet opening in its facing surface is filled with a sacrificial material that forms a top surface flush with the facing surface of the tool body. The sacrificial material is a foaming composition that forms a hard smooth skin flush with the facing surface of the tool body. Parts are made by laying a tool-side skin or laminate on the face sheathed bonding the tool-side skin to the flush surface of the sacrificial material in the peripheral groove. Honeycomb core may be placed on the skin and the assembly is bonded and/or cured with the tool-side skin conforming to the surface of the tool. After curing, the tool is removed from the autoclave and repositioned on a bed of a CNC machine tool where the honeycomb core is machined to the desired shape using a suitable cutter, and the core is vacuumed to remove the dust. The plies for a bag-side skin are-applied to the machined surface of the core and the assembly is cured. After cure, the tool is accurately relocated on the CNC machine tool bed and a peripheral edge is cut around the part using a cutter on the CNC machine tool. The controller of the machine tool is programmed to direct the cutter around the peripheral groove. The cutter projects into the peripheral groove and engages the full thickness of the part to cut the peripheral edge. After edge routing, the finished part is removed from the tool. The part stays on the tool for the entire manufacturing process, thereby eliminating the usual coordination problems that occur when the part was moved between tools for different manufacturing steps.
This invention eliminated the use of two separate very costly tools, and it eliminated much of the hand manipulation of the part previously required during removal from and positioning onto the two tools. The particular difficulty of registering large flexible composite parts onto the tool on which it was being repositioned was eliminated because the part stayed on the same tool throughout its fabrication. Likewise, a partially fabricated part having only one skin was not pulled out of shape by the stresses in the skin induced during cure, because the part remains bonded to the tool face in its original laid-up position. Subsequent machining or drilling operations on the part are performed precisely at the designated position since the part is positioned on the tool exactly where it belongs. The usual quality control procedures such as statistical process control and the like are thus now possible in configuration quality control for parts made by this process. Thus, in an environment wherein dimensional control and certainty of manufacturing parts within statistically determined tolerances is critical to the ability to manufacture products at rates that are important to the commercial success of the business, the "Dual Purpose Lay-Up Tool" invention disclosed in the above mentioned Patent Application has made a significant contribution to industrial efficiency and quality of manufacture of large composite parts.
The dual-purpose tools are made of Invar 36, an alloy of iron and nickel that has a coefficient of thermal expansion nearly identical to that of graphite-epoxy composite material. The Invar face sheet is bump-formed or cast to near net shape and attached to a "egg crate" base structure by welding. The base structure with the attached face sheet is positioned on a CNC machine tool bed and the surface of the face sheet is completely machined to provide a smooth surface with the proper profile. The machine tool is under the control of a machine controller using a part program based on the reverse of a digital model of the part to be made on the tool. The substantially continuous groove is then machined into the finished face sheet, as described in the application noted above. The resulting tool is durable and reliable, and repeatably produces parts well within the tolerances of the part specification.
The Invar face sheet on the dual purpose tools, while durable and effective for producing high quality parts, is heavy and expensive. Invar is a dense material and a large amount of it is needed to make tools on which large parts are made. The weight of such tools can exceed the load capacity of lifting and transport equipment normally available in a factory, necessitating the purchase of special lifting and transport equipment, which adds to the cost of the parts.
The manufacturing time for making dual purpose tools of Invar is long because of the time-consuming forming or casting steps and the extensive machining that is required.
The thermal mass of an Invar lay-up tool is high and can significantly prolong the time for the autoclave or oven to reach curing temperature for the composite material part laid up on the tool. The prolonged heat-up time is expensive both in terms of energy costs and in process time for making the parts.
It has been known to make lay-up tools of composite material that has the same coefficient of thermal expansion as the parts to be made. Graphite-epoxy composite tools have been made for many years, and newer materials that do not degrade at cure temperature like epoxy have become available and are becoming more widely used as their benefits become recognized. However, the art has never known a composite lay-up tool that can be produced in multiple copies and has the capability of producing a completely laid-up, cured and edge-trimmed part made entirely without removing the part from the tool.
Thus, the practice of making laid-up composite parts on "dual-purpose" tools could be greatly improved if there were available a process for producing multiple copies of dual purpose tools based on a single master tool. The copies would preferably be made quickly, inexpensively, with a high degree of precision, and have a low thermal mass to allow the cure cycle for the part in the autoclave to be as short as possible. They would be made with a process that has the capability of accurately reproducing accurately place reference features including an "A" datum plane which establishes the height and orientation of the tool face sheet over a machine tool bed, indexing features such as a "spud" and "sine key"